DE102006027357A1 - Method for operating an SCR catalytic converter and exhaust system - Google Patents

Abstract

The invention relates to a method and a program algorithm for operating an SCR catalytic converter device (16) arranged in an exhaust system (12) of an internal combustion engine (10), which comprises at least two SCR catalytic converter stages (16a, 16b) spaced apart or adjacently connected in series are suitable for at least partially storing a chemical reducing agent and reducing, with its participation, nitrogen oxides NO <SUB> x </ SUB> of the exhaust gas, the chemical reducing agent or a precursor thereof into an exhaust gas of the internal combustion engine (10) upstream of the SCR Catalyst device (16) is supplied. It is provided that a storage capacity of the first SCR catalytic converter stage (16a) in the flow direction is used as a design variable for an amount of the supplied reducing agent or its precursor. In this way it is achieved that a slip of reducing agent, in particular NH <SUB> 3 </ SUB>, by the SCR catalyst device (16) is avoided.

Description

The invention relates to a method for operating a catalyst arranged in an exhaust system of an internal combustion engine, in particular a diesel engine, which operates according to the SCR method (SCR = selective catalytic reduction). In this method, nitrogen oxides (NO _x ) contained in the exhaust gas of the internal combustion engine are reduced by involving a chemical reducing agent stored by the reduction catalyst. The invention further relates to an exhaust system suitable for carrying out the method.

In addition to carbon monoxide (CO) and hydrocarbons (HC) in particular nitrogen oxides (NO _x ) are among the environmentally hazardous, directly emitted primary pollutants, which arise during operation of internal combustion engines, especially diesel engines. A use of three-way catalysts, as used in λ = 1-controlled gasoline engines, is not possible due to the excess of oxygen in the very lean diesel engine exhaust. For this reason, a selective SCR catalyst was developed to reduce nitrogen oxide emissions in diesel engines, which reduces nitrogen oxides (NO _x ) to N ₂ and H ₂ O with an added reducing agent. The reducing agent is added directly to the exhaust gas or it is added a chemical precursor of the reducing agent, which releases the reducing agent only in the exhaust system. Ammonia (NH ₃ ), which is supplied to the exhaust gas as a gas or as an aqueous solution, serves in particular as a reducing agent. Due to the non-hazardous handling of NH ₃ , urea is usually used today as a chemical precursor, which is present either in the form of an aqueous solution or as a solid. The thermohydrolytic decomposition of urea with liberation of NH ₃ is effected by the heat of the exhaust gas stream or of the catalyst or in an evaporator or a reactor.

SCR catalysts have a temperature-dependent storage capacity for the reducing agent (NH ₃ ), whereby the temperature dependence at low catalyst temperatures is particularly pronounced. On the other hand, the NO _x conversion rate of SCR catalysts increases with the NH ₃ loading level. Therefore, it is desirable, especially at low temperatures, to keep the NH ₃ loading level as close as possible to the (temperature-dependent) maximum possible loading level. In this case, the maintenance of a safety distance to the maximum load level is common to keep the risk of reducing agent slip low.

Corresponding SCR methods and systems are from the documents DE 199 22 959 A1 . DE 100 38 741 A1 , and DE 102 51 498 A1 known, which deal with problems of supply and release of the reducing agent in the exhaust gas. The DE 199 22 959 A1 moreover discloses an SCR catalyst having two adjacently cascaded SCR catalyst stages with different NH ₃ storage capacities for achieving the most complete possible NO _x reduction.

The problem with the use of SCR catalysts are operating situations, which - in particular starting from a low exhaust gas temperature - associated with strong temperature increases, as is the case in particular with strong load jumps. These can lead to an uncontrolled release of NH ₃ stored in the SCR catalytic converter, which leaves the exhaust line unconverted. Also, such operating situations may occur when a regeneration request of the SCR catalyst upstream or downstream particulate filter is present or if a heating measure for heating a not yet operational catalyst at a cold engine start. In both cases, engine measures are initiated to increase the exhaust gas temperature, which can lead to the NH ₃ slip described. Likewise, highly exothermic processes in particulate filter regeneration or in catalytic conversion processes can lead to unwanted NH ₃ desorption. These operating conditions are more critical than for ensuring good NO _x -Konvertierungsrate loading level SCR catalyst is set with the reducing agent as possible at the maximum (temperature-dependent) level (see above).

To avoid unwanted NH ₃ breakthroughs is known to calculate the required amount of reducing agent via dosing models as closely as possible according to the amount of NO _x to be converted and fed appropriately. This is often done as a security slightly substoichiometric metering, which, however, a complete NO _{x implementation is} no longer guaranteed. Another solution provides for the use of noble metal-containing oxidation catalysts as a barrier catalyst, which are downstream of the SCR catalyst and convert occurring NH ₃ breakthroughs. However, this is associated with increased cost and space requirements. In addition, arises in a counterproductive manner in the NH ₃ conversion at the downstream oxidation catalyst again NO _x , which is undesirable on the one hand and on the other hand falsified the accounting of NO _x conversion at the SCR catalyst, if this is determined with a downstream NO _x sensor.

The invention is based on the object to provide an alternative method for operating an SCR catalyst, in which an unwanted slippage of the stored reducing agent is avoided and which is not associated with the disadvantages of the known solutions. It is also intended to provide an exhaust system suitable for carrying out the method.

This object is achieved by a method and an exhaust system with the features of the independent claims. According to the invention, an SCR catalytic converter device is used, which comprises at least two SCR catalytic converter stages (spaced apart or adjoining one another), which are each suitable for at least partially storing a chemical reducing agent, in particular ammonia NH ₃ , and, with its participation, nitrogen oxides NO _{x of} the exhaust gas to reduce. In this case, the invention provides for an absolute storage capacity of the first SCR catalyst stage in the flow direction to be used as a design variable for an amount of the reducing agent or its precursor supplied upstream of the first SCR catalyst stage alone. Control technology thus serves the absolute storage capacity of the first SCR catalyst stage as a reference variable for the reducing agent metering in the exhaust system. By neglecting the storage capacity of the second (downstream) SCR catalyst stage it is achieved that the second SCR catalyst stage is fully available to intercept any NH ₃ breakthroughs occurring in the first (upstream) SCR catalyst stage. In this case, the absolute storage capacity of the second SCR catalyst stage is preferably dimensioned such that it is ideally able to completely store the exiting NH ₃ slip of the first stage and to use it by way of the catalytic NO _x reduction. In this way, unwanted emissions of the reducing agent can be almost completely suppressed. Thus, according to the present invention, the known in the art, the SCR catalyst downstream oxidation catalyst is replaced as a barrier catalyst by the second SCR stage, which on the one hand has a cost advantage and on the other hand, takes place on the oxidation catalyst unwanted conversion of NH ₃ to NO _x .

there is in the context of the present invention under "absolute storage capacity" a maximum in the amount of reductant that can be stored in the SCR catalyst or SCR catalyst stage (e.g. in g or mol), whereas "relative storage capacity" refers to the catalyst volume a storable reducing agent level (e.g., in g / ml or mol / ml).

Basically the feeder of the reducing agent at intervals or continuously.

In a preferred embodiment of the invention, an amount of the supplied reducing agent is so dimensioned and supplied that a loading level of the first SCR catalyst stage is between a predetermined lower and a predetermined upper loading limit. In this case, the predetermined upper loading limit preferably corresponds to the absolute storage capacity of the first SCR catalytic converter stage minus a safety margin. For example, the safety margin may be chosen to be 20% of the absolute storage capacity of the first SCR catalyst stage, in particular 10% or even 5%. However, a general indication of the upper loading limit or the safety distance is not possible, since this depends very much on the design of the entire system and in particular on tolerable emission values. In principle, however, thanks to the downstream second stage, it is possible to choose the safety distance less common than in the prior art. If an NH ₃ -sensitive gas sensor is arranged between the two SCR catalyst stages (see below), the safety margin can be selected to be particularly small without risking undesirable reductant emissions. The highest possible loading of the first stage has the advantage, on the one hand to be able to use the full capacity of the first stage and on the other hand to maintain the full buffer capacity of the second stage. In this case, according to an advantageous embodiment of the invention, the aim is that the first SCR catalyst stage is always loaded with reducing agent up to the specified upper loading limit. Since the storage capacity of the first SCR catalyst stage depends inter alia on the catalyst temperature, preferably the desired loading level of the first SCR catalyst stage and / or the upper and / or lower loading limit of the first SCR catalyst stage with the reducing agent depending on a catalyst or Exhaust gas temperature predetermined. In this case, the absolute storage capacity of the first SCR catalyst stage can preferably be taken from temperature-dependent maps, whereby aging-related reductions in the storage capacity can be taken into account.

A further advantageous embodiment provides to meter the amount of reducing agent depending on an intake air mass, a predetermined by the engine control fuel mass and / or a modeled or measured (with a NO _x sensor) _NOx concentration in exhaust gas and in this sense, more or less continuously. Each of these quantities is associated with a certain measurement or determination error. This also applies to the metering itself and the efficiency for reducing agent treatment (Thermohydrolysis). In addition to these errors or uncertainties, aging effects of the SCR catalyst stage should also preferably be taken into account in the reducing agent metering.

The second (downstream) SCR catalyst stage preferably has a smaller volume than the upstream first SCR catalyst stage, in particular for reasons of space. In this case, it advantageously has a relative (ie related to the catalyst volume) storage capacity for the reducing agent (in particular NH ₃ ), which is greater than that of the first SCR catalyst stage. This can be achieved, for example, by a higher loading density of the catalyst support with storage locations, ie with the coating responsible for the storage of the reducing agent (washcoat). In this way, the second stage, despite its smaller volume, is capable of storing and reacting a relatively large amount of reducing agent (eg NH ₃ ), even under conditions in which desorption of reducing agents already takes place at the first stage. However, the (absolute) storage capacity of the second SCR catalyst stage may be less than that of the upstream first stage, since the probability of a complete Reduktionsmitteldesorption the first stage is low. For example, the absolute storage capacity of the second SCR catalyst stage may amount to a maximum of 80% of the storage capacity of the first SCR catalyst stage, in particular a maximum of 60%, preferably a maximum of 50%. Again, the size or absolute storage capacity of the second SCR stage can not be universally indicated, since this size also depends very much on the overall system design, tolerable end-uses, the dosing strategy used, and other factors.

According to a particularly preferred embodiment of the invention, the current loading level (RM_a) of the SCR catalytic converter device, in particular the first SCR catalytic converter stage, is determined continuously with the reducing agent. This is preferably done by determining the cumulative stored in the SCR catalytic converter device reducing agent quantity (RM_ΣS) and subtracting the accumulated, consumed by the _NOx conversion of the SCR catalytic converter device reducing agent quantity (RM_ΣNOx) from the stored set according to the equation RM_a = RM_ΣS - RM_ΣNOx , To determine the cumulative stored reducing agent quantity of the SCR catalytic converter RM_ΣS, the supplied reducing agent quantity is integrated over time (ie RM_ΣS = RM_ΣD, where RM_ΣD means the integrated metered reducing agent), wherein a temperature-dependent storage capacity of the SCR catalytic converter device can be taken into account. In the case of a first-stage NH ₃ breakthrough due to, for example, a sudden temperature jump, it is preferable to correct the first stage cumulative stored reductant amount RM_ΣS via the second-stage NO _x conversion. The NH ₃ breakthrough can be measured by means of an NH ₃ -sensitive sensor arranged between the two SCR stages, which can be either an NH ₃ sensor or NO _x sensor. Namely, NO _x sensors have an NH ₃ cross sensitivity, so that the NH ₃ concentration in the exhaust gas can be determined by means of suitable plausibility checks which permit a differentiation between NO _x and NH ₃ . For example, a reaction of the sensor to a change in the metered amount of reducing agent allow such differentiation.

The balancing of the NO _x conversion of the SCR catalytic converter device to determine the amount of reducing agent RM_ΣNOx consumed, that is, the comparison of the present upstream and downstream of the device NO _x content of the exhaust gas can be modeled or by using particular temperature-dependent maps or by means of upstream and / or downstream of the SCR catalytic converter device, or between the individual stages arranged NO _x sensors are measured. It is possible, for example, to model the NO _x crude emission upstream of the SCR catalytic converter device and to measure the NO _x content downstream of the SCR catalytic converter device and / or between the two SCR stages using an NO _x sensor or vice versa. The balancing of the NO _x conversion via the first and the second SCR catalyst stage is preferably carried out jointly, ie both stages are considered to be a single SCR catalytic converter.

Another aspect of the present invention relates to an exhaust system for an internal combustion engine with an SCR catalyst device comprising at least two spaced or adjacently connected SCR Katafysatorstufen each adapted to at least partially store a chemical reducing agent and its participation nitrogen oxides NO _x of To reduce exhaust gas, wherein the chemical reducing agent or a precursor thereof is supplied into an exhaust gas of the internal combustion engine upstream of the SCR catalyst device. The system has a program algorithm, which may be stored in particular stored in an engine control, for operating the SCR catalyst device of the exhaust system according to the method described above.

Further advantageous embodiments of the invention are the subject of the remaining dependent claims.

The invention will be described below in embodiments with reference to the accompanying Zeichnun explained. Show it:

1 schematically an arrangement of an internal combustion engine with downstream emission control system according to a first embodiment of the invention and

2 schematically an arrangement of an internal combustion engine with downstream emission control system according to a second embodiment of the invention.

1 shows an internal combustion engine 10 , which is a diesel engine here in particular. One from the internal combustion engine 10 coming exhaust gas is in a total with 12 directed exhaust system passed. The exhaust system 12 includes an exhaust passage 14 containing various exhaust aftertreatment components. In particular, the exhaust duct houses 14 an SCR catalyst device 16 , which according to the illustrated example, two stages, namely a first in the exhaust gas flow direction SCR catalyst stage 16a and one of these spaced downstream second SCR catalyst stage 16b , includes. For both SCR catalyst stages 16a . 16b these are selective reduction catalysts which, with the participation of a reducing agent, in this case ammonia (NH ₃ ), convert nitrogen oxides (NO _x ) of the exhaust gas into N ₂ and H ₂ O. The catalyst volume of the downstream second SCR catalyst stage 16b is significantly smaller than that of the first stage according to the illustrated embodiment 16a , On the other hand, however, the relative (volume-related) NH ₃ storage capacity of the second SCR catalyst stage ( 16b ) greater than that of the first SCR catalyst stage ( 16a ), so that nevertheless a relatively large absolute NH ₃ storage capacity of the second SCR catalyst stage 16b which, for example, accounts for about 50% of the first stage NH ₃ storage capacity 16a equivalent.

The reducing agent NH ₃ is preferably in the form of an aqueous urea solution as a chemical precursor for ammonia via a metering unit 18 , which is connected to a reservoir for the aqueous urea solution, not shown here, in the exhaust passage 14 injected. The urea solution enters a heatable evaporator 20 where the NH _{3 is} liberated by thermolysis and hydrolysis of the formed isocyanic acid. The optional evaporator 20 is advantageous because the kinetics of the urea decomposition reaction at low exhaust gas temperatures is very slow and must be expected to form undesirable polymerization. The NH ₃ thus produced becomes in the SCR catalyst 16 stored, especially in the first stage 16a ,

Instead of a urea solution, it is also possible to use solid urea (for example in the form of pellets or prills), which is processed mechanically or thermally. Thermal treatment requires a reactor outside or inside the exhaust system 12 can be arranged. In principle, gaseous NH ₃ or an aqueous ammonia solution in the exhaust system 12 be injected. Due to the problematic handling of NH ₃ and its toxic properties, however, the use of a NH ₃ -splitting precursor is preferred.

Upstream of the SCR catalyst device 16 is an oxidation catalyst 22 in the exhaust duct 14 arranged, which performs a conversion of carbon monoxide (CO) and hydrocarbons (HC). The arrangement of the oxidation catalyst 22 before the SCR catalyst 16 is particularly advantageous because, in particular, in the low temperature range, the activity of the SCR catalyst device 16 is significantly improved. Optimal in this context, if by the oxidation of NO, an NO ₂ content of about 50% based on the total nitrogen oxides NO _x behind the oxidation catalyst 22 is produced. In addition, by the upstream oxidation catalyst 22 a deactivation of the SCR catalyst 16 avoided due to increased HC emissions, which is particularly pronounced at low temperatures. Furthermore, downstream of the SCR catalyst device 16 a particle filter 24 be arranged, the soot particles of the exhaust gas filtered and is thermally regenerated from time to time. The order and design of the various components of the exhaust system 12 may differ from the one shown here. For example, the particulate filter 24 before the SCR catalyst device 16 be arranged or a particulate filter with integrated oxidation catalyst the SCR catalyst 16 be upstream.

The exhaust duct 14 also houses a number of sensors. Thus, in the case of a gasoline engine at a position close to the engine, a lambda probe 26 be arranged, in a known manner the lambda control of the internal combustion engine 10 serves. Further, according to the illustrated example, the SCR catalyst device 16 a NO _x sensor 28 downstream of which the NO _x content of the exhaust gas downstream of the catalyst device 16 measures. Optionally another _NOx sensor 28 ' between the SCR catalyst stages 16a . 16b and / or another NO _x sensor 30 upstream of the reduction catalyst device 16 be provided. In principle, NO _x sensors have a cross-sensitivity to NH ₃ , that is, the signal provided by the NO _x sensor is dependent both on the NO _x content of the exhaust gas and on the NH ₃ phthalate. Using appropriate plausibility checks, the level of the signal can be returned to NO _x or NH ₃ . For example, it is possible to check how the signal changes in response to a change NH ₃ addition (at constant NO _x content) behaves. In this way it is possible by means of NO _x sensors 28 . 28 ' and 30 optionally also to detect the NH ₃ concentration of the exhaust gas in addition to the NO _x concentration. Alternatively, instead of or in addition to the NO _x sensors 28 . 28 ' and 30 also be provided NH ₃ sensors.

The supply of the internal combustion engine 10 with air via a suction channel 32 ; in which an adjustable throttle 34 is arranged.

The signals of the gas sensors 26 . 28 . 28 ' . 30 and any other existing sensors, such as temperature sensors or other lambda probes, go into a motor control 36 one. Furthermore, by the engine control 36 various operating parameters of the internal combustion engine 10 read; For example, engine speed, coolant temperature or a pedal value of the accelerator pedal. Corresponding signal lines are shown here by broken lines. Depending on the incoming parameters, the engine control unit controls 36 the operation of the engine 10 For example, fuel injection quantities and injection timings, the position of the throttle valve 34 and the reductant metering unit 18 (corresponding control lines are shown by solid arrows).

The engine control 36 has a control unit 38 on which the operation of the SCR catalyst unit 16 controls in the manner described below. The control unit 38 includes for this purpose a program algorithm for carrying out the method according to the invention as well as necessary maps.

In the 2 shown plant is different from the one in 1 shown that the first SCR catalyst stage 16a and the downstream second SCR catalyst stage 16b adjacent to each other, that is, without spacing from each other, are arranged. This can be realized on the one hand by two separate catalyst carriers. Alternatively, the SCR catalyst device 16 however, consist of a single catalyst support bearing a different coating in its front zone than on the back zone, thereby increasing the SCR catalyst stages 16a and 16b be formed. The other elements in 2 are denoted by the same reference numerals as in 1 are therefore not explained in detail again.

The one in the control unit 38 stored program algorithm controls the SCR catalyst device 16 and in particular, the reducing agent supply unit 18 in the manner described below.

According to the invention, the absolute storage capacity of the first SCR catalyst stage alone is the reference variable for the amount of NH _{3 to be} fed into the exhaust system 16a based on. In particular, the amount of NH ₃ is metered so that one in the entire SCR catalyst device 16 stored NH ₃ amount substantially corresponding to the absolute storage capacity of the first SCR catalyst stage 16a minus a predetermined safety distance is set. In this case, preference is given to the NH ₃ charge level of the first SCR catalyst stage 16a controlled to a value between a lower and an upper load limit, both limits depending on a measured or modeled temperature of the SCR catalyst stage 16a be specified.

To control the metered addition of urea or NH ₃ is currently in the first SCR catalyst stage 16a stored NH ₃ amount continuously balanced, ie the current NH ₃ -Beladungsniveau. This is basically done by determining the accumulated, in the SCR stage 16a stored NH ₃ amount (in g or mol), determination of the cumulative amount of NH ₃ consumed by NO _x conversion (in g or mol) and finally by subtracting the consumed from the stored NH ₃ amount according to the equation RM_a = RM_ΣS - RM_ΣNOx where RM_a is the current NH ₃ charge level of the first SCR catalyst stage 16a means RM_ΣS the cumulative, in the SCR stage 16a intercalated amount of NH ₃ and _NOx conversion by RM_ΣNOx the consumed amount of NH _3.

To determine the accumulated, stored NH ₃ amount RM_ΣS is the through the feed unit 18 NH ₃ feed was determined by integration over time and a quantitative storage in the SCR catalyst 16 accepted. More specifically, the stored NH ₃ mass can be determined by additionally the temperature-dependent Einspeicherwirkungsgrad in the NH ₃ storage of the first SCR catalyst stage 16a , ie the balance of the metered to non-stored NH ₃ amount, is taken into account, with aging effects of the SCR catalyst stage 16a can be taken into account with the aid of correction variables. When desorption of NH ₃ from the first stage occurs 16a (NH ₃ -slip), roughly with the sensor 28 ' can be detected, the loading amount of the first stage is corrected accordingly.

Furthermore, the consumption of the stored NH ₃ mass RM_ΣNOx via the NO _x conversion in the entire SCR catalyst device 16 certainly. The accounting of the NO _x conversion in the SCR Ka talysatoreinrichtung 16 takes place by comparing the NO _x content upstream and downstream of the catalyst device 16 , each with the NO _x sensors 28 . 30 measured or modeled using appropriate computational models. For example, the NO _x content may be upstream of the first SCR catalyst stage 16a as NO _x raw emission as a function of a current operating point of the internal combustion engine 10 determined from stored maps or by direct measurement with the NO _x sensor 30 and the NO _x content downstream of the second SCR catalyst stage 16b with the NO _x sensor 28 be measured. Alternatively, the consumption of the stored NH ₃ mass via the NO _x conversion of only the first SCR stage 16a for which, in particular, the NH ₃ -sensitive sensor 28 ' between the steps 16a and 16b makes sense. An improvement in the accuracy of the sales accounting is achieved when possible NO _x adsorption and NO _x desorption processes in the SCR catalyst 16 or the first SCR stage 16a be taken into account. Finally, the loading level is the difference between the cumulative and SCR levels 16a stored NH ₃ -Menge RM_ΣS and the thus determined cumulative, by NO _x conversion consume NH ₃ -Mass RM_ΣNOx according to the above equation. The thus determined current NH ₃ -Beladungsniveau RM_a the SCR catalyst device 16 is in the control unit 38 even after switching off the internal combustion engine 10 stored and serves as an input for the balancing at the next engine start.

Unless the NO _x sensors 28 . 28 ' and 30 not yet reached their operating temperature and thus system readiness, which is the case in particular after a cold engine start when requesting motorized heating measures, the determination of the loading level of the SCR catalytic converter device takes place 16 until the system readiness of the NO _x sensors is reached 28 . 30 by modeling using a temperature-dependent turnover characteristic.

Does it come to a NH ₃ -slip the first stage 16a Thus, the amount of NH ₃ desorbed becomes particularly due to the high second stage NH ₃ storage capacity 16b practically completely stored by this and implemented by the nitrogen oxides of the exhaust gas. Thus, emission of unconverted ammonia can be almost completely prevented.

An improvement of the method can be achieved by detecting an exit of NH ₃ from the first SCR catalyst stage 16a by means of the interposed NH ₃ -sensitive gas sensor 28 ' the NN ₃ supply by the feeder 18 is reduced or deactivated. In this case, it may additionally be provided to adapt the metering model for the reducing agent feed in order to avoid future NH ₃ breakthroughs. This may be necessary, for example, if the NH ₃ storage capacity is reduced due to aging effects, which requires a corresponding reduction in the loading limits.

10

Internal combustion engine

12

exhaust system

14

exhaust duct

16

SCR catalytic converter device

16a

first SCR catalyst stage

16b

second SCR catalyst stage

18

supply unit

20

Evaporator

22

oxidation catalyst

24

particulate Filter

26

lambda probe

28

NO _x sensor

28 '

NO _x sensor

30

NO _x sensor

32

intake port

34

throttle

36

motor control

38

control unit

Claims (13)

Method for operating an exhaust system ( 12 ) an internal combustion engine ( 10 ) arranged SCR catalyst device ( 16 ) which comprises at least two SCR catalyst stages (spaced apart or adjoining one behind the other) ( 16a . 16b ), each of which is adapted to at least partially store a chemical reducing agent and to reduce, with its participation, nitrogen oxides NO _{x of} the exhaust gas, the chemical reducing agent or a precursor thereof into an exhaust gas of the internal combustion engine ( 10 ) upstream of the SCR catalyst device ( 16 ) is supplied, characterized in that as a design variable for an amount of the supplied reducing agent or its precursor, a storage capacity of the flow direction in the first SCR catalyst stage ( 16a ) is used. A method according to claim 1, characterized in that the reducing agent supply is controlled so that a loading level of the first SCR catalyst stage ( 16a ) between a predetermined lower loading limit and a predetermined upper loading limit, wherein the upper loading limit of the absolute storage capacity of the first SCR catalyst stage ( 16a ) minus a safety margin. A method according to claim 2, characterized in that a current loading level of the first SCR catalyst stage ( 16a ) and / or the lower loading limit and / or the upper loading limit, a loading limit of the first SCR catalyst stage ( 16a ) with the reducing agent and / or the safety margin as a function of a catalyst or exhaust gas temperature is predetermined. Method according to one of the preceding claims, characterized in that a current loading level of the SCR catalyst device ( 16 ), in particular the first SCR catalyst stage ( 16a ), is determined continuously with the reducing agent. A method according to claim 4, characterized in that the current loading level of the SCR catalyst device ( 16 ) with the reducing agent as the difference of a cumulative stored reducing agent quantity and an NO _x conversion over the entire SCR catalytic converter device ( 16 ) amount of reducing agent is determined. Method according to claim 5, characterized in that the balancing of the NO _x conversion of the first and the second SCR catalyst stage ( 16a . 16b ), wherein a NO _x content of the exhaust gas upstream and / or downstream of the SCR catalyst device ( 16 ) is modeled and / or measured. Method according to one of the preceding claims, characterized in that an exit of the reducing agent from the first SCR catalyst stage ( 16a ) by means of one between the SCR catalyst stages ( 16a . 16b ) arranged NH ₃ -sensitive gas sensor, in particular an NH ₃ sensor or a NO _x sensor, is detected. A method according to claim 7, characterized in that upon detection of an exit of the reducing agent from the first SCR catalyst stage ( 16a ) the supply of the reducing agent is reduced or deactivated. A method according to claim 7 or 8, characterized in that upon detection of an exit of the reducing agent from the first SCR catalyst stage ( 16a ) a dosage model for the reducing agent supply is adjusted. Method according to one of the preceding claims, characterized in that the chemical reducing agent is ammonia NH ₃ , the urea added from the exhaust gas is generated as a precursor or is added directly to the exhaust gas. Exhaust system ( 12 ) for an internal combustion engine ( 10 ) with an SCR catalyst device ( 16 ) which comprises at least two SCR catalyst stages (spaced apart or adjoining one behind the other) ( 16a . 16b ) Which are each adapted to at least partially store a chemical reducing agent and reduce its involvement under nitrogen oxides NO _x of the exhaust gas, wherein the chemical reducing agent or a precursor thereof in an exhaust gas of the internal combustion engine ( 10 ) upstream of the SCR catalyst device ( 16 ), characterized by a program algorithm, in particular stored in a motor control ( 36 ) for operating the SCR catalyst device ( 16 ) of the exhaust system ( 12 ) according to the method of any one of claims 1 to 10. Exhaust system ( 12 ) according to claim 11, characterized in that a relative storage capacity of the second SCR catalyst stage ( 16b ) is greater than that of the first SCR catalyst stage ( 16a ). Exhaust system ( 12 ) according to one of claims 11 or 12, characterized in that at least one of the positions upstream of the first SCR catalyst stage ( 16a ), downstream of the last SCR catalyst stage ( 16b ) and between the first and the last SCR catalyst stage a NO _x and / or a NH ₃ -sensitive gas sensor ( 28 . 30 ) is arranged.